Low “K” factor hybrid photoresist

ABSTRACT

A photoresist having both positive and negative tone components resulting in a lower “k” factor than the single tone photoresist is disclosed. The hybrid resist may either have the negative tone resist or the positive tone resist as the major portion, while the other tone is a relatively minor portion. For examples, a positive tone resist may include a minor portion of a negative tone cross-linker or a negative tone resist may include positively acting functional groups. The hybrid resist of the present invention allows for wider exposure dosage windows, therefore increasing the yield or performance and line density.

This application is a Continuation of Ser. No. 08/715,288, filed on Sep.16, 1996.

This application is a sister application to the co-pending U.S.application, Ser. No. 08/715,287 filed this same day and incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to the fabrication ofsemiconductor devices and, more specifically, to a photoresist materialhaving both negative and positive characteristics so that the processingwindows are wider.

2. Background Art

Manufacturing of semiconductor devices is dependent upon the accuratereplication of computer aided design (CAD) generated patterns onto thesurface of a device substrate. The replication process is typicallyperformed using lithographic processes followed by a variety ofsubtractive (etch) and additive (deposition) processes.

Photolithography, a type of lithographic process, is used in themanufacturing of semiconductor devices, integrated optics, andphotomasks. The process basically comprises: applying a layer of amaterial that will react when exposed to actinic energy, known as aphotoresist or, simply, a resist; selectively exposing portions of thephotoresist to light or other ionizing radiation, i.e., ultraviolet,electron beams, X-rays, etc., thereby changing the solubility ofportions of the resist; and developing the resist by washing it with abasic developer solution, such as tetramethylammonium hydroxide(“TMAH”), thereby removing the non-irradiated (in a negative resist) orirradiated (in a positive resist) portions of the layer.

As the need for higher and higher levels of integration has arisen inthe industry, the need for a larger number of lines and spaces in agiven area has increased dramatically. In response, the scaling oflithographic features has been an essential aspect of enhancing theperformance and density of semiconductor devices. Lithographic scalinghas been achieved primarily by three methods: increasing the numericalaperture (NA) of the expose tool; reducing the exposure wavelength; andimproving the response of the photoresist. These three parameters areexpressed in the Rayleigh model for lithographic resolution, as given bythe equation:

R=kλ/NA

where R is the resolution, k is an empirically derived parameter that isdependent on photoresist performance, λ is the expose wavelength, and NAis the numerical aperture of the expose tool.

The “k” factor is reduced by resists that can provide a widerfocus/expose process window for a high resolution feature. Historically,this “k” factor has been reduced by altering the resist components forexample: by adding resins and sensitizers with higher contrast;employing thinner resist films; and using anti-reflective films. Thereduction of the “k” factor is becoming more important because NA valuesare reaching their limit at 0.65-0.70 and since work at reducing theexpose wavelength from the state-of-the art of 248 nm is still inpreliminary stages.

Therefore, there is a desire for a photoresist material that overcomesone or more of the aforementioned shortcomings.

SUMMARY OF THE INVENTION

The present invention performs the functions of the usual photoresist,be it negative or positive, with less sensitivity to process conditions,i.e., a reduced “k” factor, allowing a wider range of conditions, suchas exposure dose, while still maintaining the dimensions withinallowable limits. Conversely, for a given process latitude, the smallestfeature that can be resolved in the resolution of the resist can also beimproved by using the concepts embodied in the present invention. Thesefunctions are performed by utilizing a photoresist substance thatincludes not only the traditional negative photoresist, but also“doping” the negative resist material with a proportion of a positiveresist material. In a like manner, a positive tone photoresist materialmay be “doped” with an amount of a negative resist material.

It is an advantage of the present invention that the exposure latitudemay be increased significantly, providing better control of feature sizeat all levels: lot-to-lot; wafer-to-wafer; within wafer; and withinchip. This greater level of feature control may in turn either give riseto a higher yield of product at a particular feature size because errorsare not as harmful to overall device fabrication or the feature size maybe shrunk while still maintaining line width control thus gaininggreater performance and density.

The foregoing and other features and advantages of the invention will beapparent from the following more particular description of preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and:

FIG. 1 is a graph plotting the width of lines printed using the presentinvention versus the exposure dose, thereby showing the range ofacceptable doses is greater with the enhanced k factor resist thanwithout; and

FIG. 2 is a scanning electron micrograph of lines and spaces printedusing the enhanced k factor photoresist of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Accordingly, improved, reduced “k” factor resists are provided by thepresent invention in which a first radiation induced response and asecond radiation induced response are combined in a single resistformulation and are induced by a single excitation event. For example,the first radiation induced response may be a light induced responsecausing the resist to become soluble in exposed areas while the secondradiation induced response may be the negative tone type action of acrosslinker causing the light affected areas to become higher inmolecular weight, thereby decreasing the solubility of the resistmaterial.

This concept is applicable to both positive tone systems as well asnegative tone systems. In the positive tone system formulation, whichshall be referred to as an “enhanced positive resist” to distinguishfrom the conventional positive resists, a negative acting component,such as a cross-linking agent, is used as a minor additive, therebyimparting a certain amount of negative tone chemistry to the resistformulation. Conversely, the performance or “k” factor of a negativeresist is improved by addition of an appropriate proportion of apositive acting component. Such a resist shall be referred to as an“enhanced negative resist.” It is believed that the increased exposurelatitude is obtained because the positive and negative components of thedecreased “k” factor resists counterbalance each other at the edge ofthe exposed image. As the positive tone resin is exposed to becomesoluble in the developer, the negative tone function begins tocross-link the resin and reduces its solubility. Surprisingly, thecombination provides a “buffer” against variations in exposure dosewithout a reduction in contrast or resolution capability.

The positive tone system may include as the major, active portion of theresist material any of a number of positive tone resist materials,including, but not limited to, partially protected polyhydroxystyrenes.Suitable compositions of the enhanced positive resist include thoseshown in Table 1. Examples of materials that can be employed inaccordance with this invention and their relative proportions in typicalformulations are also provided in Table 1.

TABLE 1 Components of an Enhanced Positive Resist Typical ComponentExamples Content Partially protected protecting group:  84-99% ofpolyhydroxystyrene any acid labile solids by moiety such as acetal,weight ketal or acrylate incorporated in the backbone or as a sidechain; protection typically in the range of 10 to 30% Photoacidgenerator sulfonium salts,   1-15% of onium salts, solids by triazines,dicarboximide weight sulfonates, 2,1,4-DNQ esters, dinitirobenzylsulfonates, bisaryl sulfonyl diazomethanes, imine sulfonates, arylsulfonate esters, or any other known PAG. Crosslinker multifunctional0.1-1.0% of groups such as: solids tetramethoxymethyl by weightglycoluril (Powderlink) availability from Cytec, Danbury, CT or anymultifunctional group capable of reacting with the polyhydroxystyreneSolvent propyleneglycol mono-  70-85% methylether acetate of total (PMacetate) by weight ethyl lactate, cyclohexanone or any other commonlyknown solvents

Additionally, photosensitizers and base additives may be utilized tofurther enhance the reactions of the photoresist. Sample base additivesinclude: dimethyl amino pyridine; 7-diethylamino-4-methyl coumarin(Coumarin 1), tertiary amines, proton sponge, berberine, and thepolymeric amines as in the “Pluronic” or “Tetronic” series from BASF.Additionally, tetra alkyl ammonium hydroxides or cetyl methyl ammoniumhydroxide may be used when the PAG is an onium salt.

Examples of sensitizers that may be utilized include: chrysenes,pyrenes, fluoranthenes, anthrones, benzophenones, thioxanthones, andanthracenes, such as 9-anthracene methanol (9-AM). Additional anthracenederivative sensitizers are disclosed in U.S. Pat. No. 4,371,605, whichis incorporated herein by reference. The sensitizer may include oxygenor sulfur. The preferred sensitizers will be nitrogen free, because thepresence of nitrogen, e.g., an amine or phenothiazine group, tends tosequester the free acid generated during the exposure process and theformulation will lose photosensitivity.

In accordance with preferred embodiments of this invention, the positivetone function (i.e., the deblocking chemistry) operates relatively lessdependently on a post expose bake (PEB), whereas the negative tonefunction (i.e., the cross-linking chemistry) varies in response to thePEB temperature. Therefore, the relative responses of the two tones maybe modified simply by altering the PEB temperature. In this manner, onemay optimize the extent to which the crosslinking or negative chemistryaffects the lithographic performance. Too little negative chemistry doesnot allow the benefits to be fully realized in terms of better exposuredose latitude or resolution. On the other hand, too much negativechemistry will give rise to unwanted effects such as scumming, residue,etc. in the exposed region.

EXAMPLE 1

While one particular preferred positive tone system is illustrated bythis example, it is to be understood of course, that many other examplesare within the scope of the present invention.

The following ingredients were dissolved in PM acetate solvent for atotal of 18.9% solids:

polyhydroxystyrene (PHS), available from Maruzen America, New York, N.Y.with ˜24% of phenols protected with methoxypropene (MOP), 97.5% ofsolids;

triphenyl sulfonium triflate, 1.4% of solids; tetramethoxymethylglycoluril (Powderlink), available from Cytec, Danbury, Conn., 1.0% ofsolids; and

tetrabutylammonium hydroxide, 0.1% of solids.

The resulting solution was filtered and applied to silicon wafers to athickness of approximately 0.8 μm. The coated wafers were subjected to asoft-bake of 110° C. for 60 sec and exposed with a 248 nm excimer lasersource in a 0.37 NA Canon stepper through a mask with appropriate nestedand isolated lines and spaces. The wafers were post expose baked (PEB)to 110° C. for 90 seconds and developed with 0.14 Normal (N) TMAHdeveloper for 120 seconds. The lithographic performance was comparedwith that of an identical formulation as above except with noPowderlink. FIG. 1 shows that the dose latitude of this enhancedpositive resist is about one and one-half times that of the conventionalresist as measured by positive tone response in a given dose range.

In a negative tone system, the primary photo-response is from thenegative tone resist material, however, the addition of a positive toneresist material or merely addition of groups that will function withpositive tone chemistries, such as ketal groups, again enhances theexposure latitude. Suitable components of an enhanced negative toneresist include those shown in Table 2. Example materials that can beemployed in accordance with this invention and their relativeproportions in typical formulations are also provided in Table 2.

TABLE 2 Components of a Enhanced Negative Resist. Component ExamplesTypical Content Partially protected protecting group: 75-95% ofpolyhydroxystyrene ketal, acetal, solids or a mixture oftetrahydrofuran, by weight partially protected acrylate. Protectionpolyhydroxystyrene typically in the with polyhydroxystyrene range of 10to 30% Photoacid Generator sulfonium salts,  1-15% of solids oniumsalts, by weight triazines, dicarbox- imide sulfonates, 2,1,4-DNQesters, dinitrobenzyl sulfonates, bisaryl sulfonyl diazomethanes, iminesulfonates, aryl sulfonate esters, or any other PAG CrosslinkerPowderlink or  4-10% of solids other crosslinkers by weight as listed inTable 1 Solvent PM Acetate, ethyl 70-85% of total lactate, by weightcyclohexanone or any other commonly known solvents

Again, photosensitizers and base additives, such as those listed abovemay be used to enhance the clarity of the final lines and spaces and tochange the type of radiation or wavelength at which the photoresistmaterial responds to the actinic radiation.

It should be pointed out that the difference between the enhancedpositive resist and the enhanced negative resist illustrated in Tables 1and 2 is not in the number or the type of components, however, itresides in the relative proportion of the crosslinker that is employed.In the case of an enhanced positive tone resist, the crosslinker is usedin a relatively small quantity. The purpose here is not to obtain thenegative tone response, but to slow down the positive tone in a givendose range. In the case of an enhanced negative tone resist, thecrosslinker content is relatively higher, which is typical of aconventional negative tone resists, as will be apparent to those skilledin the art. In this case, however, the negative tone resin has blockingor protecting groups that are induced by radiation to become deblocked.This deprotection reaction renders the entire resin more soluble andprovides the counterbalancing chemistry simultaneously with thepredominant negative crosslinking chemistry.

The following examples are again merely illustrative of the presentinvention and are not meant to be limiting as many other formulationsshould be readily apparent to those skilled in the art.

EXAMPLE 2

The following ingredients were dissolved in PM acetate solvent for atotal of 20% solids:

PHS, 10% hydrogenated with about 24% of phenols protected with MOP,81.2% of solids;

N-(trifluoromethylsulfonyloxy)-bicyclo-[2.2.1]-hept-5-ene-2,3-dicarboximide(MDT) available from Daychem Labs, Centerville, Ohio, 10.5% of solids;

Powderlink, 8.2% of solids; and

Coumarin 1, 0.1% of solids.

The resulting solution was filtered and applied to silicon wafers to athickness of about 0.8 μm. The coated wafers were subjected to asoft-bake of 110° C. for 60 sec and exposed with a 248 nm excimer lasersource on a 0.37NA Canon stepper through a mask with appropriate nestedand isolated lines and spaces. The wafers were post expose baked at 110°C. for 60 sec and developed with a 0.263N TMAH developer for 60 sec. Asshown in FIG. 2, the smallest line resolved in this case was 250 nm.This translated to a k factor of 0.37. When formulations were madeidentical to those above, except that the MOP protected PHS was replacedwith an unprotected PHS, and were processed in the same manner, theminimum resolution obtained was 300 nm with a k factor of 0.45. Thus,the former formulations had a k factor enhancement of nearly 20% overthe latter.

While the invention has been particularly shown and described withreference to preferred exemplary embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A composition comprising: a positive tonephotoresist material having an aqueous base soluble organic portionwherein the aqueous base soluble organic portion is partially protectedwith acid labile moieties as protective groups, wherein the degree ofprotection is sufficient to render the composition initially insolublein aqueous base and may be de-blocked to increase solubility; and aglycoluril cross-linker, wherein a percent by weight based on theorganic portion is less than about 1.0, and wherein the composition isrendered aqueous base soluble after the single excitation event eventhough the solubility of the exposed composition is reduced by theamount of glycoluril cross-linker.
 2. The composition of claim 1,wherein the aqueous base soluble organic portion of the positivephotoresist material includes phenol, and wherein the degree ofprotection from the acid labile moieties as protective groups is greaterthan or equal to 10 percent by weight of the phenol.
 3. The compositionof claim 1, wherein the acid labile moieties comprise acetals, ketals,or acrylates.
 4. The composition of claim 3, wherein the acid labilemoieties are selected from the group consisting of: acetals; ketals; andacrylates; and wherein the composition further includes a photoacidgenerator, selected from the group consisting of sulfonium salts andiodium salts, a cross-linker having multifunctional groups capable ofreacting with the partially protected polyhydroxystyrene, and a solvent.5. The composition of claim 4, further comprising a base additive. 6.The composition of claim 5, wherein the polyhydroxystyrene has aboutone-quarter of the phenols protected, wherein the acid labile moiety ismethoxypropene, wherein the photoacid generator is triphenyl sulfoniumtriflate, wherein the cross-linker is tetramethoxymethyl-glycouril, thesolvent is propyleneglycol monomethylether acetate and the base additiveis tetrabutylammonium hydroxide.
 7. The composition of claim 4, furthercomprising a sensitizer.
 8. A composition comprising: a) 84 to 99 weightpercent total solids of a positive tone photoresist material having anaqueous base soluble organic portion wherein the aqueous base solubleorganic portion is partially protected with acid labile moieties asprotective groups, wherein the degree of protection is sufficient torender the composition initially insoluble in aqueous base and may bede-blocked to increase solubility upon exposure to acid; b) 0.1 to about1.0 weight percent total solids of a glycoluril crosslinker; and c) 1 to15 weight percent total solids of a photoacid generator, wherein thecomposition is rendered aqueous base soluble after a single excitationevent.